EARLY ARC MAGMATISM: GEOCHEMICAL CHARACTERISTICS OF VOLCANIC CLASTS FROM PUNTA SÁMARA, COSTA RICA

Abstract

The geochemical characteristics of volcaniclasts from Punta Sámara are used to infer magmaticprocesses that generated Late Cretaceous – Middle Paleocene arc lavas in the western margin of the Caribbeanplate. The Ba/La ratios of the volcaniclasts are higher than the ratios for any of the modern lavas from Costa Rica, and the La/Yb ratios are lower; suggesting that the lavas from the primitive arc were generated by greatercontribution of slab material than the modern arc lavas. Furthermore, the lower La/Yb ratios imply that thedegree of melting was significantly higher and/or that the mantle source was more depleted than in modernCosta Rica. The low Zr/Nb ratios for the modern lavas from central Costa Rica imply a more enriched mantlesource (OIB-like), whereas the higher ratios for the primitive arc volcaniclasts imply a more depleted mantlesource (MORB-like). We use the geochemical information to infer the conditions of subduction during the earlystages of arc magmatism in the southern part of the Central American arc. We infer that the primitive arc wasassociated with a subducting slab that dip at a steeper angle than the present. The steeper angle would imply thatthe subducting Farallon plate was older, colder and denser than the present Cocos plate.